1. Remote sensing techniques have been applied to augmentrnestimation of hydrologic variables. Thus rainfall, evaporation, floodedrnzone delineation and runoff estimation has been carried out. TRMMrnmonthly rainfall products were employed in rainfall gap fillingrntechniques, the satellite estimates were first validated against observedrnground station records and results indicated good correlation, averagernof 81%. Combination equation (Energy balance-mass transfer) is usedrnto determine gridded potential evapotranspiration from the sub-basinrnby making use of MODIS 16 days Albedo products. Spatial extent andrnamount of water on flood plains during wet seasons of the year has alsornbeen determined by making use of spot NDVI temporal images throughrncalibrating with high resolution SAR and optical images. Furthermore,rnland use/land cover information is extracted from Landsat imagery andrnexisting data of land cover has been updated. Using the update landrncover information spatial Curve Number (CN) map was developed.rnRunoff estimation, using SCS method and a soil water balancernsimulation using WatBal model, was then carried out in order torndetermine the runoff from un-gauged catchments and the results fromrnthe two models were compared. Results indicated that the soil waterrnbalance simulation model gave better correlation with the observedrnthan the SCS method. Therefore, runoff estimation from un-gaugedrncatchments was carried out using the soil water balance simulationrntechniques. Lake water balance simulation was finally carried out usingrncontinuity equation within simulation time span of 1998 to 2003rnconsidering the variables, i.e. inflow estimation form the contributingrncatchments, the improved evaporation estimation; the rainfall datarnobtained incorporating TRMM images and water lost in the flood plainsrndue to evaporation. Simulation results were compared with the LakernTana Storage computed using the recorded Lake water surfacernelevation. The six year average results indicated that out of the total ofrn7,688 MCM estimated runoff from the catchments 3.59% is lost in thernflood plains due to evaporation. Similarly, out of the 10,574 MCMrninflow to the lake (net runoff plus rainfall over lake area) 5,110 MCM isrnlost through evaporation from the lake surface, 5,012 MCM is outflowrnfrom the lake to the head of main Abbay river. Average reduction inrnreservoir storage was computed to be 330 MCM. Therefore, thernremaining 1414 MCM is assumed to be seepage loss and other unaccountedrnlosses from the system